What was done
The authors investigated the thermosteric (i.e., due to temperature only) sea-level change over the last 50 years using the global ocean temperature data sets of Levitus et al. (2000) and Ishii et al. (2003).

What was learned
It was found that thermosteric sea level variations are dominated by decadal oscillations of the planet's main coupled ocean-atmosphere climatic perturbations (El Niņo-Southern Oscillation, Pacific Decadal Oscillation and North Atlantic Oscillation).  In terms of the global mean, in the words of the authors, "thermosteric trends computed over 10-year windows exhibit a time oscillation of ~20-year period," and they say that these trends "show large fluctuations in time, with positive values (in the range 1 to 1.5 mm/year for the decade centered on 1970) and negative values (-1 to -1.5 mm/year for the decade centered on 1980)."

What it means
Over the full half-century period of data, there has been a net rise in sea level due to the thermal expansion of sea water, but only because the record begins at the bottom of a trough and ends at the top of a peak.  In between these two points, there are both higher and lower values, so that one cannot be sure what would be implied if earlier data were available or what will be implied as more data are acquired.  Noting that sea level trends derived from Topex/Poseidon altimetry over 1993-2003 are "mainly caused by thermal expansion" and are thus "very likely a non-permanent feature," Lombard et al. conclude that "we simply cannot extrapolate sea level into the past or the future using satellite altimetry alone."  Consequently, even the 50 years of global ocean temperature data we possess are insufficient to tell us much about the degree of global warming that may have occurred over the past half-century, as any long-term increase in global thermosteric sea level it may have caused is absolutely dwarfed by decadal-scale variability.

References
Ishii, M., Kimoto, M. and Kachi, M.  2003.  Historical ocean subsurface temperature analysis with error estimates.  Monthly Weather Review 131: 51-73.

Levitus, S., Stephens, C.M., Antonov, J.I. and Boyer, T.P.  2000.  Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948-1998, U.S. Government Printing Office, Washington, DC.